function res = fn_ff4(k, kc)

res = 2/3/pi/(1+4*k^2)/kc^3 ...
    +8*k/pi/(1+4*k^2)^2/kc^2 ...
    +8*(48*k^4+8*k^2-1)/pi/(1+4*k^2)^4/kc ...
    +16/pi*(16*k^4-24*k^2+1)/(1+4*k^2)^4*(pi/2+atan(2*(k-kc))) ...
    +64*k/pi*(4*k^2-1)/(4*k^2+1)^4*log((1+4*(k-kc)^2)/(4*kc^2));

end